1. Field of the Invention
The present invention is directed to an indicating device having at least two sealed chambers arranged in tandem directly behind one another in either the direction of travel of light in the device or the direction of viewing with adjacent chambers being separated by thin carrier members with at least one of the chambers being filled with a liquid crystal layer to form a liquid crystal cell which cooperates with the other chamber to provide indications. In several embodiments, different types of liquid crystal cells are used such as a polarizing cell is combined with a rotation cell to enable multi-color indication. In other embodiments, the electrode patterns of each of the cell are arranged to enable either selective indication of fixed symbols by separately actuating one cell or by actuating electrodes in both cells to give a complete indication.
2. Prior Art
Indicating screens or devices utilizing liquid crystals have many applications. For example, a liquid crystal cell formed by a layer of liquid crystal material disposed between two plates with each of the plates having a pattern of electrodes have been suggested. The electrodes on one plate are parallel strips and the electrodes on the other plate are parallel strips extending at 90.degree. to the strips on the first plate to form an electrode matrix for applying a field to discrete areas of the liquid crystal cell. When producing a sign pattern with such an indicating screen, problems occur with this type of matrix since undesired cross points of the stips of the matrix will be energized to energize undesired portions of the cell while attempting to energize the desired portions of the configuration of the sign. To overcome these disadvantages, prior art devices have energized the various strips of the electrode matrix in a time sequence utilizing the storage capability of the layer which is dependent on the relaxation time of the liquid crystal after the electrical field is removed to give a complete picture from a series of energized areas.
The storage characteristics of the liquid crystal due to the relaxation time of the crystal to re-assume its condition when no electric field or potential is applied has prevented the use of liquid crystal cells to produce animated displays in which portions of images are moving and particularly, a moving image display in which portions of the moving parts overlap the preceeding image.
Instead of using a matrix of parellel strips with the strips on one surface of the layer extending perpendicular to the strips on the other surface, it has been suggested to provide indications of numbers by utilizing segmented electrodes which are separately energized in distinct patterns to form or indicate various numerals. While such an indicator screen has many applications, it cannot be used in certain applications, for example, the field of railroad signaling techniques or for scale. This is due to a lack of redundance of the indication of the sign. For example, to determine if a correct pull or weight has been indicated, it is impossible to detect the current being applied to individual segments of the liquid crystal cell since the current is so small and is strongly dispersed. Thus, there is no way of determining whether the proper segments are being energized.
In addition to display screens utilizing either an electrode matrix or segmented electrodes, display screens have been suggested which enable presenting either a colored indication on a light or white background or a light or white indication on a colored background. Such a device is discussed in an article by George H. Heilmeier, Joseph A. Castellano and Lewis A. Zanoni entitled "Guest-Host Interactions in Nematic Liquid Crystals", Molecular Crystals and Liquid Crystals, Volume 8, 1969, pages 293-304. This article discusses mixing a pleochroicdye such as a dichroic dye in a nematic liquid crystal layer to form a liquid crystal cell. When light which has been polarized strikes the cell or passes through the cell, the dye molecule if aligned in the direction of polarization will absorb light and exhibit a characteristic color of the particular dye. However, if the orientation of the dye molecule, i.e. their long axis, extends perpendicular to the direction of the electric vector of the polarized light, the light is transmitted through the cell unchanged. The article discusses applying a field to the liquid crystal layer to change the orientation of the layer and dye molecules to change the color of the light passed by the cell.
Liquid crystal cells can be of different types. One type of liquid crystal cell has a liquid crystal material which is a nematic material with a positive electric anisotropy and is a so-called rotational cell (TN). Another type of cell has a nematic material with a negative electric anisotropy and has a homeotropic orientation and is sometimes referred to as a DAP cell with DAP referring to the deformation of aligned phases.
It is suggested in an article by Susumu Sato and Masanobu Wada entitled "Liquid-Crystal Color Display By DAP-TN-Double-Layered Structure" IEEE Transactions on Electron Devices, May 1974, pages 312 and 313 to place a rotational cell (TC cell) and a separate DAP cell between a pair of cross polarizers. The DAP cell will transmit only one color tone from a white light passing through the arrangement. However, as pointed out by this article, an application of an electric field on the TN cells changes the direction of polarization of the light passing through the DAP cell to produce a different color. However, the arrangement as suggested in this article requires two complete individual cells which have high production cost and the device may suffer from parallax faults.
Another utilization of a liquid crystal indication screen is proposed in an article by Ian A. Shanks entitled "Liquid Crystal Materials and Device Developments" Electronic Engineering, August 1974, pages 30-37. In the article, a multi-colored presentation is suggested and achieved by placing two cells in tandem with a composite birefringent layer between the cells. This arrangement is placed between two polarizers which are either cross polarizers or parallel polarizers. Each of the cells is a rotational cell which will rotate the direction of polarization with the amount of rotation of the two cells being different, for example .pi./2 radian and .pi./4 radian. As disclosed in the article, by selectively actuating the two cells, four different colors can be achieved. It is suggested that the cell could be utilized with a cathode ray tube to convert a black and white picture into a colored picture image. In the device disclosed, the construction has three comparative thick layers between the two liquid crystal layers which may cause problems with parallax and the device is expensive to produce.
In a copending application of Helmut Katz, entitled "A Compound Liquid Crystal Indicator Screen" U.S. Ser. No. 545,108, filed Jan. 29, 1975, which issued as U.S. Pat. No. 3,992,082, a compound liquid crystal indicator screen having two liquid crystal cells arranged in tandem directly behind one another in the direction of travel of light through the device was proposed. This device included a stack of spaced members sealed together at their margin by glass solder with the stack including a pair of outer glass plate members with at least one transparent, very thin carrier member interposed therebetween to form the spaced chambers, which receive the liquid crystal layers and are separated from adjacent chambers by a carrier member. The construction of this device provides a compound liquid crystal indicator device which was substantially free of parallax.